Exhaust gas discharged from internal combustion engines of vehicles, etc., or combustion engines, such as boilers, contains hydrocarbon (HC), carbon monoxide (CO), nitrogen oxide (NOx), and like hazardous substances that cause air pollution. Efficient purification of these hazardous substances is an important issue in terms of, for example, prevention of environmental contamination. Exhaust gas purification techniques that can purify these three hazardous substances at the same time have been actively studied. For example, there is a known technique of purifying the three hazardous substances at the same time by the catalytic function of catalysts capable of purifying hazardous substances provided in internal combustion engines or combustion engines. Such catalysts are generally called three-way exhaust-gas catalysts, and some of them have already been put into practical use.
In recent years, it is desired for three-way exhaust-gas catalysts for automotive applications to efficiently purify exhaust gas even when the concentration of hazardous substances (CO, HC, and NOx) rapidly changes from several ppm to several percent. That is, there is an increasing demand for three-way exhaust-gas catalysts that can flexibly respond to rapid changes in the concentration of hazardous gas. Moreover, the system for purifying hazardous substances using such a catalyst is desired to maintain the oxygen concentration at a predetermined level or higher. This is because oxygen present in exhaust gas promotes the oxidation of CO and HC, and the reduction reaction of NOx, and thus the function of the purifying system is further improved by maintaining the oxygen concentration at a constant level. However, for vehicles, it is difficult to maintain a constant engine combustion state because the running state varies depending on, for example, the degree of road congestion. The oxygen concentration of the exhaust gas also changes continuously. If the oxygen concentration decreases, the performance of purifying hazardous substances by the catalyst is also reduced, thereby making it difficult to build a high-performance purifying system. From such a viewpoint, in recent methods, a compound having an oxygen storage capacity (hereinafter referred to as “OSC”) is mixed into exhaust gas purifying catalysts as an OSC agent. Since the OSC agent can supply oxygen when the oxygen concentration of the exhaust gas is low, it is easier to prevent a decrease in the function of purifying hazardous substances in the purifying system.
Ceria powder is known as an OSC agent. It is clarified that the use of ceria powder, which has a relatively large OSC, as an OSC agent for exhaust gas purifying catalysts increases the efficiency of treating exhaust gas. Various proposals have been made so far for ceria-based powder whose oxygen storage capacity and oxygen release properties are improved, such as ceria-zirconia-based composite oxides, and exhaust gas purifying catalysts using the ceria-based powder as a co-catalyst.
For example, PTL 1 discloses a composite oxide in which a solid solution is formed from ceria, a metal or an oxide of at least one element selected from zirconia and iron, and a metal or an oxide of at least one element selected from silver and praseodymium. This composite oxide does not use noble metals, such as platinum, and is thus provided as an inexpensive catalyst. However, expensive elements, such as silver and praseodymium, are used in the catalyst disclosed in PTL 1 to impart catalytic activity; thus, there is a disadvantage that cost reduction is difficult.
Moreover, PTL 2 discloses a purifying catalyst comprising catalyst powder consisting of a transition metal oxide, and an oxygen release material. This purifying catalyst contains iron as the transition metal oxide, cerium as the oxygen release material, and zirconium as an inorganic oxide, and has a purifying action even though a noble metal is not used as an essential component. However, this purifying catalyst has limited efficiency of purifying exhaust gas.
Furthermore, PTL 3 discloses an exhaust purification catalyst comprising a particle containing a noble metal and a particle containing ceria and zirconia carrying an iron compound. This exhaust purification catalyst has high NOx conversion efficiency, and uses a less amount of noble metal than conventional catalysts. However, even though the amount of noble metal used can be reduced, it is necessary to use noble metal elements, such as rhodium; thus, this catalyst is disadvantageous in terms of cost.
PTL 4 discloses an auxiliary catalyst for cleaning flue gas in which a noble metal is supported on a composite oxide of CeO2, ZrO2, and a metal oxide that does not react with CeO2 or ZrO2, wherein the composite oxide has a pyrochlore phase where Ce and Zr are regularly arranged. In this composite oxide, the metal oxide intervenes between ceria and zirconia in the composite oxide as a barrier, which suppresses grain growth. The composite oxide thus has a high specific surface area. In particular, the composite oxide exhibits a high OSC because of the pyrochlore phase where Ce and Zr are regularly arranged. However, the catalytic activity of the composite oxide is likely to decrease after a heat treatment at a high temperature, and high temperature durability is problematic.
In addition, PTL 5 discloses a catalyst system comprising exhaust emission control catalysts including a first catalyst supported on an inorganic structure carrier, and a second catalyst (cerium-zirconium-based composite oxide) different from the first catalyst. PTL 5 also discloses that the cerium-zirconium-based composite oxide, which is the second catalyst, can be produced by heating and melting a mixture of raw materials of the composite oxide at a temperature equal to or higher than the melting point thereof, cooling the molten material to thereby obtain an ingot, and grinding the ingot. The catalyst system using exhaust emission control catalysts including such a composite oxide can particularly efficiently purify NOx. However, even this exhaust emission control catalyst has limited purification efficiency, and there is still a room for further improvement.